1. Field of the Invention
This invention relates to imaging systems and spectroscopic systems using nuclear magnetic resonance. In a primary application the invention relates to deriving NMR information from regions of the body with improved signal-to-noise ratio.
2. Descripton of Prior Art
In magnetic resonance imaging system, as presently practiced, the dominant noise source is the body losses, far exceeding both the coil losses and the first amplifier noise. As a result, to improve the SNR, surface coils have been designed which couple to only a portion of the body, to reduce the resultant noise. These are described in a book entitled Nuclear Magnetic Resonance and Its Applications to Living Systems, by D. G. Gadian, Clarendon Press, Oxford, 1982. Unfortunately, these coils provide improved performance for only superficial parts of the body, close to the surface. Also, it is difficult to control the shape of the sensitive volume which is essentially prescribed by the diameter of the coil as described in a paper by C. E. Hayes and L. Axel entitled, "Noise Performance of Surface Coils for Magnetic Resonance Imaging at 1.5T," Medical Physics, 12:604-607, Sept. 1985. As a result, if it is desired to view an interior region with high SNR, or perform localized spectroscopy on an interior region, no method is presently available.
In the 1985 Preceedings of the Society of Magnetic Resonance in Medicine, London, two papers were presented which showed surface coils using pairs of coupled coils in several configurations. Although these configurations showed some desirable properties they still concentrated on superficial structures. In each case the maximum sensitivity was exhibited at the surface of the body, with the sensitivity monotonically decreasing in deeper regions. Thus deeper lying regions are received with relatively poor SNR since these coils are also deriving noise signals from a region near the surface. These combination of coils were not structured to cancel undesired regions.